Evaluation of a ⁴²Ca-⁴³Ca double-spike for high precision Ca isotope analysis

Abstract

Use of a ⁴²Ca-⁴³Ca double-spike for Ca isotopic analysis on Thermal Ionization Mass Spectrometers (TIMS) offers two important advantages over the ⁴²Ca-⁴⁸Ca or ⁴³Ca-⁴⁸Ca double-spikes that are currently used to correct for instrumental mass fractionation according to the exponential law. First, since the normalizing (⁴²Ca/⁴³Ca) and the corrected (⁴⁴Ca/⁴⁰Ca) ratios differ in their average mass by only 0.5 mass unit, they will tend to fractionate coherently thereby minimizing errors in the corrected ratio due to any deviation of the actual machine fractionation from an exponential law. Secondly, all four isotopes (40-44) required for the analysis can be measured simultaneously in modern multi-collector TIMS. This will eliminate errors in mass fractionation correction caused by rapid fluctuations or drifts in fractionation and will also reduce analysis time substantially. We report here initial data for laboratory standards and seawater using a ⁴²Ca-⁴³Ca double-spike that show how these advantages lead to more consistent and faster Ca isotopic analysis than hitherto possible.